Polysaccharides such as cellulose, starches, alginates, and pectins are extremely important commercially both in industrial applications and in consumer products. There is, therefore, a growing interest in the preparation of saccharide-derived monomers that can be used to prepare addition-type polymers containing saccharide functionality. The efficient preparation of saccharide-derived monomers has proved to be challenging however.
Simple elaboration of one of the available saccharide hydroxyl groups into a polymerizable group cannot, in general, be used to prepare mono-functional monomers because it is difficult to chemically distinguish the multiple hydroxyl substituents. For example, esterification of sucrose with methacryloyl chloride gives a mixture of sucrose mono-methacrylates and some sucrose dimethacrylates. The presence of even a small percentage of difunctional monomer can lead to the formation of gels upon polymerization, which in most cases is undesirable. A number of different approaches have been taken to either circumvent this problem or otherwise selectively prepare mono-functional saccharide-derived monomers.
The use of protecting groups to block the reactivity of the excess hydroxyl groups is a classic strategy for the synthesis of saccharide-derived monomers. There are numerous reported syntheses in which the saccharide to be converted into a monomer is per-acetylated, and then a polymerizable functionality is elaborated onto the anomeric carbon. Similarly, a protecting group strategy can be used to mask all but one of the hydroxyl groups, which can then be esterified with an acrylic or a methacrylic acid derivative or converted to an allyl ether. In the latter case, the allyl ether can be hydrosilated with poly(dimethylsiloxane-co-methylsiloxane) to give a polysiloxane with pendant saccharide functionality. The principal drawback of the protecting group strategy is the need to introduce and then remove the protecting groups.
Other workers have sought to elaborate the anomeric carbon into a polymerizable group using a glycosidase enzyme catalyst. The enzyme .beta.-galactosidase was used to transfer lactose or o-nitrophenyl .beta.-D-galactopyranoside to 2-hydroxyethyl acrylate or 2-hydroxyethyl methacrylate. Apparently, the desired galactopyranoside monomers could be obtained without using protecting groups. A potential drawback of this synthesis is that the saccharide is attached to the monomer (and, later, the polymer backbone) by an acid sensitive glycosidic linkage.
Enzymes have also been used to selectively esterify a single hydroxyl of a saccharide. For example, lipase or protease enzymes have been used to transesterify an active ester of acrylic acid such as vinyl acrylate with saccharides or saccharide derivatives. High conversions to predominantly monofunctional monomer in which only the primary hydroxyl of the saccharide is acryloylated are reported.
An alternative strategy to selectively esterifying a single hydroxyl group with an acrylate ester is to start with a saccharide molecule that contains a single functional group that is either significantly more reactive than a hydroxyl group, or reacts in a different manner. One such approach involves the condensation of amino saccharides such as glucosamine or 1-amino-1-deoxy-D-glucitol with acrylic or methacrylic anhydride. Exclusive formation of the corresponding (meth)acrylamide is expected because of the high reactivity of the single amino substituent relative to the many hydroxyl groups.
Another approach that exploits saccharide functionality other than hydroxyl groups involves the condensation of amine containing monomers with aldonic acid lactones. For example, condensation of p-vinylbenzylamine with .delta.-glucono-lactone gives N-p-vinylbenzyl gluconamide. The great advantage of this approach is that difunctional monomer is not a possible side-product of the synthesis. Primary amine containing monomers, however, are not readily available commercially.
Various alkyl-amino substituted mono- and disaccharides have been obtained by reductive amination of reducing mono- and disaccharides with C.sub.4 -C.sub.10 alkyl amines. Subsequent coupling of the alkylamino substituted mono- or disaccharides with a vinyl substituted isocyanate was carried out at low temperature in aqueous systems or in organic solvents. However, due to the nature of the vinyl moiety present in the substituted isocyanate, these saccharide monomers can homopolymerize, which is undesirable in many cases.
It is apparent that significant efforts have been expended to develop polymerizable, saccharide-derived monomers and methods for preparing same. However, the methods and/or monomers disclosed heretofore are either impracticable, or suffer deficiencies of crosslinking due to their multifunctionality, or can homopolymerize. In certain applications, it would be desirable, then, to prepare polymerizable, saccharide-derived monomers which are substantially monofunctional and which will not homopolymerize.